\documentclass[tikz]{standalone}
%\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
  \pgfmathsetmacro{\lensRadius}{2}
  \pgfmathsetmacro{\lensHeight}{1}
  \pgfmathsetmacro{\startAngle}{asin(\lensHeight/\lensRadius)}
%  \pgfmathsetmacro{\stepSize}{5}
  \tikzstyle{pt}=[circle, draw, fill=black,inner sep=1pt];

  \coordinate (shiftY) at (0,0.3);
 % \coordinate (shiftX) at (0.3,0);  
  \coordinate (shift) at (2,0);
  \coordinate (shiftA) at (0, 1.5);

% n,n+1 points coordinates
  \node (v1)[pt] at (-2,-1) {}; 
  \node (v2)[pt] at (-2,1) {};
  \node (v4)[pt] at (3,1) {};    
  \node (v5)[pt] at (3,-1) {};
% point labels
\draw
  ($ (v1) - (shiftY) $) node {$x_{2,n}$}
  ($ (v5) - (shiftY) $) node {$x_{2,n+1}$}
  ($ (v2) + (shiftY) $) node {$x_{1,n}$} 
  ($ (v4) + (shiftY) $) node {$x_{1,n+1}$}; 
% convex arrows
 \draw [open triangle 45-] (-2,\lensHeight) arc[start angle=180-\startAngle,delta angle=2*\startAngle,radius=\lensRadius];
 % \draw [-open triangle 45] (-2,\lensHeight) arc[start angle=\startAngle,delta angle=-2*\startAngle,radius=\lensRadius];

 \draw [open triangle 45-] (-2+5,\lensHeight) arc[start angle=180-\startAngle,delta angle=2*\startAngle,radius=\lensRadius];
 %\draw [-open triangle 45] (-2+5,\lensHeight) arc[start angle=\startAngle,delta angle=-2*\startAngle,radius=\lensRadius];

\draw [-open triangle 45]{(v4)--(v1)};

% dashed lines coordinates
  \coordinate (v1-) at ($ (v1) - (shift) $);
  \coordinate (v2-) at ($ (v2) - (shift) $);
  \coordinate (v4+) at ($ (v4) + (shift) $);
  \coordinate (v5+) at ($ (v5) + (shift) $);
  
% draw lines
%upper
  \draw [dashed] {(v2-)--(v2)};
  \draw (v2)--(v4) 
node[pos=0.25, pt]{} 
node[pos=0.5, pt]{}
node[pos=0.75, pt]{};
  \draw [dashed] {(v4)--(v4+)};
%lower
  \draw [ dashed] {(v1-)--(v1)};
  \draw (v1)--(v5)
node[pos=0.5, pt]{};
  \draw [ dashed] {(v5)--(v5+)};


% time axis and labels
\coordinate (t) at ($ (v5+) - (shiftA) $);
\coordinate (tn0) at ($ (v1) - (shiftA) $);
\coordinate (tn1) at ($ (v5) - (shiftA) $);

  \draw [-triangle  45] {($ (v1-) - (shiftA) $)-- (t)};
  \node [pt] at (tn0){};
  \node [pt] at (tn1){};
  \node at ( $ (tn1) - (shiftY) $ ) {$t_{n+1}$}; 
  \node at ( $ (tn0) - (shiftY) $ ) {$t_n$}; 
  \node at ( $ (t) - (shiftY) $ ) {$t$}; 
% H label
  \draw [<->]{ ($ (tn0) + 0.5*(shiftY) $) --( $ (tn1) + 0.5*(shiftY) $)};
  \node at ( $ (tn0) !.5! (tn1) + 1.5*(shiftY) $ ) {$H_{n}$};   
  
% h labels
\coordinate (p1) at ($ (v2) !.25! (v4) $);
\coordinate (p2) at ($ (v2) !.5! (v4) $);

\draw [<->]{ ($ (p1) + 0.5*(shiftY) $) --( $ (p2) + 0.5*(shiftY) $)};
\node at ( $ (p1) !.5! (p2) + 1.5*(shiftY) $) {$h_{x_1}$};

\draw [<->]{ ($ (v1) - 0.5*(shiftY) $) --( $ (v1) !.5! (v5) - 0.5*(shiftY) $)};
\node at ( $ (v1) !.25! (v5) - 1.5*(shiftY) $) {$h_{x_2}$};

  
\end{tikzpicture}
\end{document}
